Keel on foam boat

Discussion in 'Boat Design' started by BCer, May 1, 2018.

  1. BCer
    Joined: May 2018
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    BCer Junior Member

    I have been working on a 12 foot (11' 6" LOW) foam sailing/paddling leisure boat. The beam is 34". The glassed over foam is very sturdy. More then expected. Originally, the intent was to build a trimaran where amas with 'wings' would provide needed lateral resistance. Since it is a flat bottom vessel, the stability should be great in flat water, I am considering launching the rig and test the stability. Cat rig, Lateen sail size 10 x 10 x 10.5 feet made out of tarp until I get the right size.
    My question now is about the daggerboard.

    Would a full length 4" keel suffice to provide enough lateral resistance? It would be still light and would not interfere with shallow water use.

    Any ideas would be much appreciated.
     
  2. rwatson
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    rwatson Senior Member

    That's not ideal.
    Even a 12 ft mirror sailing dinghy has a dagger board with more area.

    A long narrow keel will make steering problematic
     
  3. BCer
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    BCer Junior Member

    Thank you.
    Regarding the daggerboard area: I used 4% formula of sail area as the guideline. That calculation points to 253 sq inches or lateral resistance. Over 10' length that would amount to just over 2.5" keel, Considering that full keel would interfere with steering, what would you say about tho parallel 4 foot long 3" tall/deep keels?
    That amounts to 288 sqin, 4.5% of sail area.
    I do not foresee any heeling issues: the vessel if flat bottomed, I am heavy enough to counter the wind force an keep the boat flat on the water.
     
  4. messabout
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    messabout Senior Member

    If you hope to sail to windward, then give the boat a proper board of some kind. Four percent is a minimum. The long runners will not have the same affect as a dagger board of equal area. Even with four foot long "runners" your turning ability will suffer. With a board of......say 12 inch chord sticking down below the bottom two feet you will have nearly adequate lateral resistance. If the board is located correctly with respect to the sail center of effort the combination will work well enough.

    At 34 inch chine beam, the boat will be a bit tender when under sail. That is especially true if the boat is a double ender. The flat bottom is the most initially stable for a given width but 34 inches is a bit narrow, even for use with such a small sail. An expert sailor would do alright with this layout. An inexperienced sailor might get some unplanned thrills....wet ones. I say go for it but do pick your weather carefully until you get a feel for the boat.

    You can probably get a used Opti pram rig pretty cheaply. It is somewhat smaller than the lateen you are contemplating. A decent used Pram sail will probably drive the boat as well or better than the larger home brewed tarp sail.

    Post some pictures of your boat
     
  5. BCer
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    BCer Junior Member

    Hi, Thank you for the thorough explanation. I was hoping to avoid making floats and akas, but it seems like that a multi hull would be the best course of action given the issues with the boat's shape and size. I am going for 8' foam floats, with a 3' plywood fin in the middle. Since my hull is 10" tall, with the estimated draft of about up to two inches. I will firs try straight akas with floats 10" in height (including the fins).

    The challenge with foam hull is that any mounts resisting a pulling force are fairly weak. I have explored a few options and it seems that the most effective (both cost and strength wise) is the use of the webbing straps mounted around the hull. With straps 'hugging' the hull from the bottom all around the pulling force that can be applied to exceeds any realistic values. Akas would rest on plywood beds while being tied down to the straps. The same goes for the traveler line (the design feature of Sunfish). The main sheet block might be connected to the hull in the same way.

    I will post some pictures. The boat is still work in progress as I am about to finish the sanding and strapping.
     
  6. messabout
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    messabout Senior Member

    A conventional boat with 11.5 feet of water line and 34 inch chine beam is likely to displace a total of something like 175 pounds at 2 inch draft. You said above that you are heavy enough to keep the boat flat. Do your calculations again to determine the probable draft and also to decide what the bottom rocker is best advised.
     
  7. BCer
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    BCer Junior Member

    No floats or float hardware included in the calculation below:
    The flat bottom surface is 26 sqft
    Total hull volume is 22 cubic feet (including the cockpit)
    Total bouyancy based on the foam properties 22x52= 1150 lbs of bouyancy
    Main hull should weigh roughly 30 lbs, with the rest of the rig it should weigh roughly 55 lbs.
    Weight of boat itself should float on 55/52 (1.06) cubic feet of hull
    Empty vessel draft is 12/26, .46 inches.
    For each additional 55 lbs of weight the draft would increase for another hafl inch. The projected crew and gear weight is roughly 250 lbs, total probable draft of 3 inches.
     
  8. BCer
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    BCer Junior Member

    The boat is currently in sanding / shaping phase:
    1. Need to set the mast sleeve - 3" ABS pipe (the mast is 10' spruce linearly tapered from 3" @ 8" to 2" @ 10'
    2. Once the mast sleeve is in the mast bow pieces can be secured, including the top
    3. Straps installed on (around) the hull: at least one for traveler, one for the aft aka, one for the fore aka
    4. Glass over (leaning towards using 8 - 10 oz cloth with Titebond II as I had success with this method in the past)

    More photos will follow after this.
    IMG_20180502_221554.jpg IMG_20180502_221612.jpg IMG_20180502_221659.jpg
     
  9. messabout
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    messabout Senior Member

    The middle picture of the transom suggests that you have no rocker in the after part of the bottom. If that is the case the boat will drag pretty badly. If it was a double ender (that is pointed at both ends), you could use little or no bottom rocker, even though that would make it harder to turn.

    The dimensions of the mast are robust, actually overkill for a stick to carry the small sail you anticipate.

    You have made an interesting boat BCer. I would try it as it is before adding amas. ...But I would first put some rocker in the bottom before it is too late. If the bottom area at the chine is actually 26 square feet the boat will be a bit more stable than I first predicted. It will have more wetted surface than a boat of similar dimensions however.
     
  10. BCer
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    BCer Junior Member

    So how much of rocker do you have in mind? If 2" is enough, another layer of foam on the bottom will give enough material to scrape out a nicely curved bottom. I have already applied some bottom curvature - about a foot length at transom as well as about a foot at the bow. The mast is an overkill, yes, but since it has been built from two pieces of wood (limited by the size of lathe) and connected by a half lap splice it needed some 'meat' to maintain strength as the join is in the middle and the splice length is 14".
     
  11. messabout
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    messabout Senior Member

    The general idea is to raise the bottom, at the transom, sufficient to rise above the water surface by a small amount. That implies that the bottom of the boat will have a curve when viewed in profile (elevation view). The question; how much curve is not immediately evident. Some calculation is needed. You must have a pretty good estimate of the depth of immersion (draft) of the hull when fully loaded. As soon as you put some curvature in the bottom the displacement estimate becomes a little bit more involved. The hull is no longer a simple geometric figure.

    I do not mean to make this a complex explanation but a bit of math is involved. Nothing daunting like differential calculus............Simple arithmetic. The usual drill is to do a scale drawing of the boat in both the plan view and the elevation view. Divide the length of the boat into some number of sections. Think of it as bread slices at defined locations. Choose a number of sections, evenly spaced, that will yield an odd number of locations. In your 12 foot boat you could use 12 inch spacing on your drawing. Lay down a preliminary waterline on the elevation view of your drawing. Your 12 foot boat, at about 250 pounds total displacement, is likely to have something like 3.5 to 4 inches of draft. Put the waterline at say four inches above the base line. Base line is a reference line on which the lowest part of the bottom will rest. Spring a batten that will trace the preliminary bottom curve. Draw a horizontal line there. That is the bottom of your boat. Use a batten to draw the sheer line of the boat in plan view and also the chine line if you intend to give the sides some flare.

    You now have access to the two dimensions that will be under water when the boat is in use. Make a chart with five or six columns labeled Breadth, depth, product, Simpson, product S. Measure each of the widths and depths of the boat's drawing at the various section locations. Multiply width B and depth D, to find the immersed area at each of the stations. In the fourth column write 1 (one) at the first location, 4 at the next location, then 2, than 4 then 2 and so on until at the next to last station you will have a 4. The last station will be labeled 1 (one) Those little numbers are Simpson factors. Multiply the area figure that you have in the third column by the Simpson factors in the fourth column. Place the product of those numbers in the fifth column. Now ! Add up all the figures in the fifth column, multiply that sum by the space between the sections. Divide that product by the number 3 (three). Now you have a pretty good estimate of the actual displaced volume of the wet part of the boat. Lets say that the result of all that yields ..........something like 6900. That is the number of cubic inches of displacement and you can determine the buoyancy by multiplying by the constant 0.0361 Doing that you will have determined the displacement in pounds as 250. Where did that constant come from? Fresh water weighs 62.4 pounds per cubic foot. A cubic foot has 1728 cubic inches so divide 62.4 by 1728 = 0.03611. That is the number of pounds for each cubic inch of displacement.

    With all that you can determine just how much rocker to put into the bottom of the boat. Get the transom a wee bit above the waterline. Use the batten to draw a smooth curve. The boat will track better if you let the bow be slightly immersed. If you do that the boat may be inclined to root in a chop or wake. I would let the bow at the forefoot, just touch the waterline. The boat will turn better that way but not be as directionly stable.

    I do hope that the above is not too much to process at one time. Do the homework. It will be well worth the time and effort. When you do the drawing use a fairly large scale like 3 inches to the foot. For a twelve foot boat the drawing will be 36 inches long. If you use a smaller scale the dimensions lifted from the drawing can not be as accurate. Rip some battens from a softwood like spruce or straight grained fir. about 3/16 by 3/8 will make a good size for such a tool. When springing the batten, place the weights that hold the battens in place away from the ends of the boat. Play with the weight location and you can see differences in the curves that the batten will describe. Have fun.
     
  12. sharpii2
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    sharpii2 Senior Member

    It would have to be more like 4+ 3/8 inch deep. It would still not be great for upwind use, only adequate. You may be looking at 120 degree tacks or more. The rudder can be deeper if it can kick up or if it is easily dismountable. but don't make it too deep or the boat will tend to pivot around it.

    A 11.5 ft long, 4+ 3/8 inch deep keel will have approximately 4.4 sf, by my calculation. And this is the number my comments are based on.

    I also assume 1ft free board and about 50 sf of sail.

    It will resist quick turning, as others have noted, so may not be able change tacks without backing the sail and/or using a paddle to assist.

    It my be possible to forgo a keel entirely, as your boat seems to have sharp chines and vertical sides.

    The trick, in this case, is to move the rig far enough aft so the rudder itself acts as the lateral area. This is an ancient trick used by Arab dhows to North Thumberland cobles. The rudder, in this case, will have to be quite deep and had better be able to kick up. The proposed lateen sail would work well in this case, as its Center of Area (CA) is well aft the mast.
     
  13. BCer
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    BCer Junior Member

    The sail center of effort will be at 69 inches from the bow. Pretty much at the middle of the boat. Wouldn't that be too far if the rudder assumes the function of the keel?
     
  14. BCer
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    BCer Junior Member

    Regarding the rocker design calculations - you have provided a great guide. By taking away material from the bottom to create the curvature there would be some impact on the draft. In order to maintain the above the water heigth a new layer needs to be added at the bottom before the rocker can be carved out.
     

  15. messabout
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    messabout Senior Member

    Do whatever is needed, within reason. Yes, by introducing some rocker in the bottom you will have removed some amount of potential displacement. In that case the required depth of immersion will have increased. Lets estimate..... Think of the area of the largest section of the boat. It is width times breadth....as long as you don't start getting fancy with beveled chines, vee bottoms, or rounded ones. In any case the area of that section gives us some clues about tinkering with the various dimensions of the boat.

    The main section of the boat, the one with the largest immersed area, can be compared with the average section area taken from the sum of all the areas and dividing by the number of sections. Divide the average section area by the largest or main section area. The result of the division, the quotient, will be less than one. A boat of the type that you are dealing with will have that ratio somewhere near 0.55 If the boat was actually a box shaped vessel the same width and depth at all stations, then the ratio would be 1.00 because all the sections have the same area. Here is a quicky method for getting the approximate displacement of your boat or conversely determining the depth of draft.

    Multiply the area of the main section by the length of the presumed waterline. Make a SWAG estimate that the draft is going to be four inches. and the width is 34 inches.....4 x 34 = 136 square inches.....Now convert that to the estimated ratio of 0.55....here we go....136 x 0.55 = 74.8. we have just made a wild guess at the average area of all the sections including the main section. Multiply the figure. 74.8 by the length of your waterline.....in inches... which is 138. Here goes, 74.8 x 138 = 10,322 cubic inches. Use the constant 0.0361 times 10.322 = 372 pounds. Whoops.... we have over estimated the draft. For your total displaced weight of 250 pounds we need 250/0.0361 = 6923 cubic inches of displacement. Our chine width remains the same so we must reduce the area of the main section by reducing the draft... 6923/10322 = 0.67 so we must have a draft of 4 inches time 0.67 or 2.68. This is a fairly useful wild assed estimate of what the rocker ought to be . Set the bow at about two and a half and the stern at about three to three and a half inches. of rocker.

    When you make a scale drawing of the boat, set the rocker at somewhere near those numbers and observe that the length of the waterline has probably changed. Now you need to go back through the numbers and do the draft estimate again. One of the numbers, the waterline length, has changed. No problem your hand held calculator can do the arithmetic in a matter of seconds. Keep doing this until you are satisfied that you have it right, or at least in the ball park. Also be aware that the ratio mentioned above may not always be the one I have suggested. A boat of the sort that you are building will almost always be somewhere between the 0.50 and the 0.60 displacement ratio. Skinny boats like kayaks are in the lower range and boxy ones are in the higher range.

    Big, really big, disclaimer: This method can get you into the ball park with the nearly accurate figures. It is not even close to the more esoteric methods used by real naval architects. It works well enough for we amateurs who are only interested in really small boats. I repeat, Have fun.
     
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